Electrophoresis is a well developed chemical analysis technique. A review reference on this subject is Chapter 9 of Chromatography-Fundamentals and Applications of Chromatographic and Electrophoretic Methods, Part A: Fundamentals and Techniques, edited by E. Heftmann, Elsevier Scientific Publishing Company, 1983, herein fully incorporated by reference. Capillary electrophoresis (CE) is an important advance in electrophoresis which was pioneered by Jorgenson and Lukacs as reported in Analytical Chemistry, 53, 1298-1302 (1981) and in Science 222, 266-272 (1983), each of which are herein fully incorporated by reference.
In capillary electrophoresis, the electrophoretic movement of a charged analyte species is augmented or inhibited by the bulk electroosmotic flow (EOF) of the electrolyte medium. In conventional systems, both electrophoretic and electroosmotic movement results from the same applied voltage and cannot be independently controlled.
In most CE applications, some degree of stacking occurs. Stacking means that ionic solutes injected in the sample volume become concentrated in a lesser volume prior to separation. The shape of the concentrated sample plug after stacking controls the efficiency of the eventual separation of the sample constituents. For a cylindrical capillary, the ideal form is that of a cylindrical plug. The stacking process itself originates from a difference in the electric field between the sample zone, which is typically less conductive than the bulk electrolyte, and the bulk separation medium in the capillary. However, the EOF is also different between the sample zone and the bulk separation medium; this ensures that the sample will not be stacked as a perfectly cylindrical plug, but will be parabolically distorted at the front or rear edges.
In systems where detection preferably takes place outside the high electric field (as in suppressed conductivity detection and some types of electrochemical detection), flow in the post separation zone cannot be plug like. It has been reported that augmentation of EOF by a hydrostatic head or pressure minimizes the loss of separation efficiency in such systems. (See, for example, Kok, W. Th. Anal. Chem. 1993, 65, 1853-1860.)
A different approach to controlling EOF in a capillary is based on the magnitude of the EOF in a capillary being a function of the zeta potential of the capillary surface which can be manipulated by a second, radially applied, field. (See, for example, Ghowsi, U.S. Pat. No. 5,092,972.) The magnitude of the radial field can be maintained uniform across the length of the capillary using a resistive coating or a liquid medium surrounding the outer side of the separation capillary to uniformly dissipate the second field. The radial field can also be applied more simply through a conductive sheath; in this case, the radial field is not uniform across the capillary. In either case, little or no plate losses occur; a significant amount of additional work has been carried out to further develop the concept of a radially applied field in CE. (See, for example, Ewing, A. G. Anal. Chem. 1993, 65, 2010-2013.) The major drawback of the radial field application approach is that the degree of control that can be exercised over the EOF is highly pH dependent and can be very limited at pH values far removed from the pK.sub.a of the surface ionizable groups.
The art of capillary electrophoresis would be improved by an apparatus capable of controlling EOF with less dependence on pH and capable of performing CE analysis with minimized distortion of the sample plug.